This invention related to processes for removing sulfur oxides from gas mixtures such as flue gas containing the same.
Sulfur dioxide is a constituent of various waste gases. Among these ae flue gases formed by the combustion of fossil fuels, off gases from various chemical and petroleum refining processes, and smelter gas. A major source of sulfur dioxide pollution of the atmosphere is flue gas from electric power plants. Such flue gas typically contains about 0.2-0.3% by volume of sulfur dioxide (more or less, depending on fuel sulfur content), trace amounts of sulfur trioxide, and some oxygen (e.g., about 1-6% by volume) due to the use of excess air in combustion. Sulfur dioxide is irritating to the respiratory system, is toxic to plant life, and is corrosive to metals. Hence, it is imperative that discharge of sulfur dioxide into the atmosphere be held to a low level.
Various processes have been suggested for the removal of sulfur oxides from gases. These may be classified generally into wet and dry processes. The present invention is concerned with the latter. Dry processes generally employ a regenerable solid sorbent comprising an active material for the selective removal of sulfur oxide from gases on a porous support or carrier. The active material generally comprises a metal or metal oxide, such as copper or copper oxide, or a potassium oxide-vanadium pentoxide mixture, and the porous carrier is generally an essentially inert material such as alumina. Dry processes for flue gas desulfurization using solid sorbents are disclosed, for example, in U.S. Pat. Nos. 3,411,865 and 3,501,897, and in British Pat. Nos. 1,089,716 and 1,160,662. Copper oxide on alumina, which is the sorbent composition disclosed in British Pat. No. 1,089,716, is a preferred flue gas desulfurization sorbent. Desulfurization is accomplished by passing flue gas containing sulfur dioxide through a bed of sorbent until the effluent SO.sub.2 concentration reaches a predetermined level, then regenerating this sorbent with a reducing gas. For example, when removal of 90% of the sulfur dioxide in the entering gas is desired, desulfurization is stopped and the sorbent is regenerated when the cumulative amount of SO.sub.2 in the effluent gas over a whole sorption period reaches 10% of the amount of SO.sub.2 in the incoming gas. During desulfurization (or sorption), a part of the active material is sulfated, i.e., converted into a metal sulfate. Thus, for example, copper oxide is partially converted to copper sulphate. This sorption-regeneration cycle can be repeated a large number of times before the sorbent requires replacement.
The active material of the sorbent ordinarily is not completely utilized. That is, when the sulfur dioxide content in the effluent gas reaches the predetermined level, there is still unconverted metal oxide in the sorbent. Furthermore, it has been found that, in general, there is a negative correlation between the strength maintenance of the sorbent and the extent of sulfation of the active material which can be achieved. In other words, the best sulfations are generally obtained with the sorbents which show the greatest loss in crushing strength and hence the greatest susceptibility to attribution with repeated cycling. Another consideration in selecting a suitable flue gas desulfurization sorbent is that effective sulfur oxide removal must be achieved with a low pressure drop, since flue gas in a conventional power plant is ordinarily available at a pressure only slightly above atmospheric.